refactor: VectorWriter without nVersion

The field is unused, so remove it.

This is also required for future commits.

src/wallet/test/wallet_tests.cpp

@@ -752,7 +752,7 @@ bool malformed_descriptor(std::ios_base::failure e)
 BOOST_FIXTURE_TEST_CASE(wallet_descriptor_test, BasicTestingSetup)
 {
     std::vector<unsigned char> malformed_record;
-    CVectorWriter vw{0, malformed_record, 0};
+    VectorWriter vw{malformed_record, 0};
     vw << std::string("notadescriptor");
     vw << uint64_t{0};
     vw << int32_t{0};